The interface between an integrated circuit and external circuitry is commonly a large complementary metal-oxide-semiconductor (CMOS) inverter. A CMOS inverter is well known in the art and generally includes a p-channel field effect transistor (PFET) and an n-channel field effect transistor (NFET) connected in series with their drains tied together and the output taken from the common drain. The two gates may be tied together and the input applied to the common gate. This circuit, known as an output driver, drives a capacitive load by rapidly switching the load. The output load may comprise a data bus and circuits attached thereto.
Various challenges are presented in the design of output drivers and related components. For example, large CMOS output drivers can consume large amounts of current during output transitions, thus leading to poor power efficiency. To increase power efficiency, some output driver designs have included a break-before-make (BBM) predriver. The BBM predriver may be used to control the gate inputs to the CMOS inverter individually so that the transistor driving the load is turned off (break) before the other transistor is turned on (make). This approach tends to reduce or eliminate crowbar current. Crowbar current is an undesirable effect which may result if both the PFET and NFET of the output driver are on.
However, even with the advantages BBM predrivers provide, challenges exist in the absence of good control over predriver output signals. For example, where significant delays exist between the make and break edges on rising and falling transitions, the efficiency of the output driver is generally decreased. Moreover, also because of issues related to the efficiency of the output driver, it is sometimes desirable that the delay between the input and make edges be equal for rising and falling transitions, resulting in a good duty cycle. It is also sometimes desirable that the delay between the break and make edges are equal for rising and falling transitions. Accordingly, flexibility in the number of buffered stages may be provided because the inverted version of a signal could be used at each predriver output, and each predriver output could be interchangeably connected to the PFET or NFET of a following output driver. Substantially equal delays between the make and break edges for rising and falling transitions may also assist in decreasing duty cycle distortion.
Voltage level-shifting may also be desirable for BBM predrivers for output drivers. Output drivers often drive an I/O device. These I/O devices may operate at higher voltages than the remainder of the chip, including the output driver driven by the BBM predriver. Accordingly, it may be desirable to increase the voltage level input to I/O device. Voltage level-shifting provides one way of increasing the voltage level input to I/O devices.
Tristate capabilities may be desirable for output drivers. Output driver circuits may include two transistors that assume in low-impedance either a high (logic “1”) state, or a low (logic “0”) state. In essence, these transistors are always either on, i.e., in a conductive state, or OFF, i.e., in a nonconductive state. It is often required by input-output (I/O) design specifications that the output driver be capable of disconnection from a data bus. The disconnection of the output driver for a period of time is advantageous because several I/O devices might be linked to the same output data bus.
A third state is sometimes used to disable a single output driver that is connected to a data bus having plural output drivers. Devices capable of existing in this third state may be known as “tristate” devices. The non-driving output drivers may be disabled and placed in the high impedance state such that the non-driving devices will not interfere with the output of the driving device. Because this third state may be desirable for the output driver, it would also be advantageous to have a predriver that incorporates tristate capabilities.
There is a need for a BBM predriver device that provides good control over signals, thereby decreasing duty cycle distortion.
There is further a need for a BBM predriver device that incorporates voltage level-shifting capabilities. Level-shifting would allow for more compatible operation of the predriver with I/O devices that may be driven by the output driver that is, in turn, driven by the predriver.
There is further a need for a BBM predriver device that incorporates tristate capabilities, allowing the predriver to be disabled when its corresponding I/O device is connected to the same data bus as other I/O devices.